Method of degassing hydrogen from solid titanium



United States tion of Delaware N Drawing. Filed Feb. 13, 1958, Ser. No. 714,961

. 3 Claims. (Cl. 148- -13.1)

This invention relates to purification of titanium, including alloys of titanium with minor amounts of other metals, and is more particularly concerned with a process for removing hydrogen from titanium.

Titanium readily picks up hydrogen, and the hydrogen content has been found to increase from beginning toend of titanium fabrication operations. Thus descaling is accomplished by reducing oxide scale in a fused bath containing sodium hydride and then pickling with. an acid to remove the reducedscale, both of which cause the titanium to pick up hydrogen. The reaction with the acid during pickling generates hydrogen. Annealing is accomplished in a reducing atmosphere which also introduces hydrogen. Additional hydrogen will be picked up in any caustic baths employed. Titanium is seriously embrittled by hydrogen, but no economically practical way has been found to avoid this hydrogen absorption during fabrica-' tion. Furthermore, the industry is closely allied with that of alloy steel fabrication, so it is desirable to use the same treatments for both, insofar as possible. An effective process for removing hydrogen from titanium is obviously desirable.

It is an object of this invention to provide a process for removing hydrogen from titanium and its alloys. Anotherobject is to provide such a process for removing hydrogen which also removes scale, making a separate descaling treatment unnecessary. A further object is to provide a process suitable for removing hydrogen from titanium and its alloys after a conventional descaling treatment, such as a sodium hydride descaling treatment used for ferrous alloys. Other objects will become apparent from the specification and claims.

In accordance with the present invention hydrogen is removed from titanium, including alloys of titanium with up to 10% of other metals, by treating the titanium with calcium metal in an inert molten bath of an inorganic salt or mixture of salts at a temperature of 500 to 850 C. The treatment should be sufficient to removeany coating on the titanium, in order for the removal of absorbed hydrogen to be etfective. has little or no scale and has not been chemically descaled, treatment at 500 C. to 700 C. for as little as 5 minutes is adequate to reduce the hydrogen content to about parts per million or less. When the titanium has a thick scale or the scale has been removed by treatment with sodium hydride or other chemical action, then treatment at about 700 C. or higher, and longer treatment times, are desirable for adequate removal of hydrogen. Greater thicknesses of titanium may also require higher temperatures than thin titanium, in addition to longer treating times. Treatment at about 700 C. for about minutes is suitable for removing hydrogen to depths of up to at least 0.1 inch below the surface of the titanium unless there is an unusually thick scale.

The only requirements for the molten bath are that it be molten at the desired temperature, be inert to titanium, and dissolve at least 0.01% by weight of calcium metal. A variety of suitable inorganic salt baths are already known to the art, of which calcium chloride is preferred.

i atenr When the titanium lCC Since calcium chloride meltsat about,7l.0 C. it should be mixed with sufficient sodium chloride or other inert flux to give a molten bath at lower treatment temperatures. A melt containing 15v mole percent. of sodium chloride (very approximately 10, weight percent) is suitable for use at 700 C. The melt should contain from 0.01% by weight of calcium metal up to the maximum amount which will dissolve in the melt, substantial saturation being preferred. The'calcium metal can be added directly to the melt or can be formed in the melt by reaction of an alkali metal with the calcium chloride, as by adding metallic sodium. In either case oxidation of the metal should be prevented by providing aninert atmosphere over the melt. A protective atmosphere of argon is preferred, but other non-oxidizing gases can be used, such as helium.

After treatment of titanium with the melt containing calcium the titanium is preferably quenched in water, and pickled to clean the surface. A brief pickling of about one minute is sufficient to clean the surface without causing objectionable hydrogen pick-up. A conventional pickling bath containing nitric and hydrofluoric acids is suitable. The treatment can also remove scale, so previous descal ing operations are often unnecessary. However, the treatment is preferably used after the conventional descaling with sodium hydride followed by pickling in an aqueoussolution of nitric and hydrofluoric acids.

In the following examples, which illustrate specific embodiments of the invention, parts refers to parts by weight unless otherwise specified:

Example 1 A mixture of 625 parts of calcium chloride and 252 parts sodium chloride was melted in an atmosphere of argon and saturated with calcium at 540 C. Type A titanium (apart from its hydrogen contentan industrially pure form of titanium), in sheet form 0.015 inch thick, was immersed in this melt for 15 minutes at 540" C. The titanium was then pickled for 1 minute at 6070? C. in a conventional industrial pickling bath, used for removing oxides and nitrides from the surface of titanium. This bath was prepared by mixing 234 volumes of water,

60 volumes of concentrated itric acid and 6 volumes of- 50% hydrofluoric acid. The titanium was descaled and the hydrogen content was reduced from 200 parts per million to less than 10 ppm. by this treatment. I

The time of treatment in the melt is not criticalsince the hydrogen was likewise reduced to less than 10 p;p.m. by immersion times of 5 to 60 minutes. This permits the melt to be used for annealing the titanium. i

The calcium chloride melt need only contain suflicient sodium chloride to make it molten at the temperature used. Calcium metal is an essential ingredient of the bath. When Example 1 was repeated with no calcium in the bath there was no reduction in the hydrogen content of the titanium. The calcium can be added as such or can be formed in the melt by adding metallic sodium, whereupon the following reaction occurs:

2Na+CaCl 2 Ca+2NaCl When the bath becomes spent, it can be revived by further additions of calcium or sodium.

Example 2 A mixture of 625 parts of calcium chloride and 252 parts sodium chloride was melted at 540 C. in an atmosphere of argon and saturated with sodium. Specimens of 0.015 inch thick A 70 titanium were immersed in the melt for different treatment times, pickled as in Example s r nes 1, and. then analyzed for hydrogen with the following results:

Annealing scale was removed from the titanium by the treatments of Examples 1 and 2. However, a more drastic treatment is required for the thicker mill scale. This can be accomplished by a preliminary descaling with a conventional sodium hydride treatment, followed by pickling, and then treating in a melt of higher temperature than previously illustrated;

Example 3 The titanium to be treated was industrially pure A 70 titanium, hot rolled to 0.134 inch thick and thickly coated with mill scale. in a conventional hydride bath, containing 1 to 2% Nail-I in molten NaOH, at 400 C. for minutes. The titanium wasthen pickled for 1 minute at 6070 C. in a bath prepared by mixing 234 volumes of water, 60 volumes of concentrated nitric acid and 6 volumes of 50% hydrofluoric acid. A melt was prepared by heating a mixture of 625 parts of calcium chloride and 252 parts of sodium chloride to 690 C. in an atmosphere of argon and saturating the melt with sodium. The titanium was immersed in this melt for minutes and then again pickled for 1 minute in the above pickling bath. The titanium was descaled by this treatment. and analyzed parts per million of hydrogen. The weight loss caused by the treatment was 0.002 to 0.005 gram per square centimeter per hour, which is about the same as that caused by conventional sodium hydride and pickle descaling, and the appearance of the metal was equally good.

The preliminary descaling of Example 3 can be omitted, even when the scale is quite thick, by using somewhat higher melt temperatures, e.g., about 800 C., for the calcium treatment.

Example 4 The titanium was descaled and the hydrogen content was decreased from 260 p.p.m. to 14 p.p.m.

This was given a preliminary descaling Example 5 The treatment of Example 4 was repeated, but using a calcium chloride melt saturated with sodium at 800 C. in an atmosphere of argon. At this temperature, preparation of the melt in this way was less desirable than that of Example 4 because sodium vapor distilled out in the argon and about 30 minutes was required to liberate the required amount of calcium. The titanium was descaled by the treatment and analyzed 15 p.p.m. of hydrogen.

Example 6 A titanium alloy containing 5% aluminumand 2.5% tin, commonly designated A AT, in sheet form 0.043 inch thick, was treated by the procedure of Example 3. The alloy was descaled and the hydrogen content was reduced from 320 p.p.m. to less than -10 p.p.m. by the treatment.

Example 7 A titanium alloy containing 8% manganese, commonly designated C 110 M, in sheet form 0.045 inch thick, was treated by the procedure of Example 3. The alloy was descaled and the hydrogen content was reduced from p.p.m.'to l2 p.p.m.

Since many different embodiments of the invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited by the specific illustrations except to the extent defined in the following claims.

We claim:

1. The process for removing hydrogen from titanium,

including titanium alloys, which comprises treating solid titanium with calcium metal in an inert molten bath of inorganic salt at a temperature of 500 to 850 C.

2. The process for purifying solid titanium which cornprises melting a mixture of calcium-chloride and inert flux at 500 to 850 C., incorporating at least 0.01% by weight of calcium metal in the melt, applying the melt to the surfaces of the titanium at 500 to 850 C. and then cleaning the surfaces to produce titanium substantially free of hydrogen and scale.

3. A process as defined in claim 2 wherein the calcium metal is liberated in the melt by adding sodium metal to the melt.

References Cited in the tile of this patent UNITED STATES PATENTS 2,546,320 Rostron Mar. 27, 1951 2,790,738 Alexander et a1 Apr. 30, 1957 2,807,539 Quin Sept. 24, 1957 FOREIGN PATENTS 158,458 Australia June 12, 1952 164,967 Australia Jan. 29, 1953 

1. THE PROCESS FOR REMOVING HYDROGEN FROM TITANIUM INCLUDING TITANIUM ALLOYS, WHICH COMPRISES TREATING SOLID TITANIUM WITH CALCIUM METAL IN AN INERT MOLTEN BATH OF INORGANIC SALT AT A TEMPERATURE OF 500* TO 850*C. 